1. Field of the Invention
The present invention relates to an image recording apparatus provided with an exposing head that controls the exposure light to a photosensitive material in accordance with an image signal. In particular, the present invention relates to an image recording apparatus which can correct nonuniformities in exposure light amounts outputted from the exposing head. The present invention also relates to a method for recording images.
2. Discussion of the Related Art
Various image recording apparatuses, which are called digital line exposure devices, have been proposed. This type of image recording apparatus records images on paper by taking images on film or the like as image data, and exposes the paper to light in accordance with the image data obtained from the film.
Exposure control in an image recording apparatus is usually performed by an exposure unit. The exposure unit is comprised of, for example, an exposing head, a light source and an optical fiber bundle for introducing light from the light source to the exposing head. The exposing head is provided with shutter portions (light output portions) for individually controlling pixels (dots), which are linearly arranged in a direction (widthwise direction of the paper) normal to the transport direction of the paper, so as to block or unblock the transmission of the light.
When a drive voltage corresponding to the image data is applied to the shutter portion in a position corresponding to a desired pixel, the shutter portion is opened, and the light from the light source having been introduced to the shutter portion is emitted toward the printing paper, thereby exposing the paper. In this way, a desired pixel image can be recorded.
In the above described image recording apparatus, light output characteristics of the respective shutter portions of the exposing head change with time. This results from, for example, deterioration of the light source, partial variations in the transmittance of light through the optical fiber bundle, and a reduction in the function of the exposing head. With such changes in the light output characteristic over time, variations in exposure light amounts among the respective shutter portions become larger. As a result, exposure quality is reduced over time, and accurate images cannot be reproduced.
Accordingly, in known image recording apparatuses, a sensor for measuring the exposure light amounts of the respective shutter portions is provided in a position facing the exposing head so as to enable scanning along the direction in which the respective shutter portions are aligned. Based on the measured light value of the sensor, shutter opening times can be adjusted for each shutter portion. In other words, with this construction, even if the light output characteristics of the exposing head change over time, a reduction in image quality can be avoided by correcting the exposure light amounts based on the measured results from the sensor.
Alternatively, there is a method according to which a high resolution scanner for the original image is provided instead of the above light sensor. After a solid image of a single color is recorded on the entire surface of the printing paper, the printing paper is scanned by the scanner, and the amount of exposure light is corrected in accordance with the scanned result of the original image. According to this method, a test chart in which the respective shutter portions and the pixel positions of the printing paper have a one-on-one correspondence may be recorded by exposure.
Generally, the depth of focus is small in a digital line exposure device. Thus, in order to maintain good exposure quality of the exposing head, displacement of the printing paper in the focusing direction (direction normal to an image recording surface of the printing paper) is permissible in a range of 0.05 mm. Accordingly, when a sensor is provided opposite to the exposing head, a low-precision sensor having a scanning displacement of e.g. 1.00 mm (0.50 mm: this means a displacement range of 1.00 mm displaced 0.50 mm from a proper scanning position) is insufficient. Therefore, a high precision sensor having a scanning displacement of 0.10 mm (0.05 mm) or smaller, preferably 0.04 mm (.+-.0.02 mm) or smaller is required to achieve satisfactorily high precision. It is noted that scanning displacement is a type of index indicating the precision of the sensor and is actually an indication of the displacement of the sensor during scanning in the normal direction.
If the exposing head can handle printing paper having a width of up to 305 mm, which is becoming mainstream, the scanning distance of the sensor needs to be 305 mm or longer. However, the longer the scanning distance, the more difficult it generally becomes to maintain a scanning displacement of 0.1 mm (.+-.0.05 mm). Even if such a scanning displacement can be achieved, the production of the apparatus becomes more complicated, which increases the price of the apparatus. Thus, in order to achieve cost effective scanning, it is reasonable to use a sensor having a scanning displacement of 1.00 mm (.+-.0.50 mm).
However, an apparatus using a sensor having a scanning displacement of 1.00 mm (.+-.0.50 mm) has the following problem. FIG. 3A shows a measurement curve when the exposure light amounts of the respective shutter portions were measured by a sensor having a scanning displacement of 0.04 mm (.+-.0.02 mm). FIG. 3B shows a measurement curve when light amounts were measured by a sensor having a scanning displacement of 1.00 mm (.+-.0.50 mm). In this example, the exposing head had an exposure resolution of 400 DPI (dots per inch). The exposing head includes, for example, 5120 light output portions in order to handle printing paper having a width of 305 mm, accounting for errors in the width of the printing paper. It should be noted that Dot No. 1 and Dot No. 5120 correspond to the shutter portions located at the opposite ends of the linearly arranged shutter portions of the exposing head.
As can be seen from FIGS. 3A and 3B, a portion judged as having substantially proper amounts of exposure light by a sensor having a scanning displacement of 0.04 mm (.+-.0.02 mm) may be judged to be insufficient using a sensor having a scanning displacement of 1.00 mm (.+-.0.50 mm) due to the displacement of the sensor. In such a case, a controller generates a correction value to increase the exposure light amount, i.e. a correction value different from the correction value actually needed. Accordingly, the exposure correction cannot be accurately performed when using a sensor having a scanning displacement of 1.00 mm (.+-.0.50 mm). As a result, prints of high quality cannot be obtained.
On the other hand, when the exposure correction is performed using a high-resolution scanner, it is necessary to use a scanner having a resolution at least higher than the exposure resolution of the exposing head. It is known that a scanner having a resolution five times that of the exposure resolution must be used in order to obtain a satisfactory correction value. In other words, an original scanner having a resolution of 2000 DPI or higher is needed to achieve an exposure resolution for the exposing head of 400 DPI. However, since such a high-resolution original scanner is expensive, the price of the image recording apparatus becomes too expensive if the exposure correction is performed using this method.